As a conventional capacitance type pressure sensor, there has been known a configuration, as shown in FIG. 3. The conventional capacitance type pressure sensor includes a detection capacitor formed of a diaphragm and a fixed electrode, the diaphragm being deformed under application of a pressure, and it further includes a fixed capacitor that is connected in series with the detection capacitor. In this configuration, a rectangular wave voltage is applied to these capacitors and it is intended to detect a divided voltage applied to the detection capacitor (Patent Literature 1). By detecting the divided voltage applied to the detection capacitor in this way, the pressure applied to the diaphragm can be measured.
In specific, an output voltage obtained based on the divided voltage applied to the detection capacitor is detected by an amplifier (first-stage amplifier) and an output voltage of the first-stage amplifier is synthesized and converted to a DC voltage using an inverting/noninverting circuit (including an analog switch). Thus, it is configured to calculate the pressure based on a value of the synthesized DC voltage.
In this capacitance type pressure sensor, the diaphragm is grounded via a frame (referred to as “frame-grounded” hereinafter) and the detection circuit such as an inverting/noninverting circuit is grounded by a signal connection (referred to as “signal-grounded” hereinafter) in order to give a reference potential.
In the conventional capacitance type sensor, assuming that a frame-grounded potential (referred to as “FG potential” hereinafter) is equal to a signal-grounded potential (referred to as “SG potential” hereinafter), a divided voltage is detected based on a following expression.Vdiv(ac)=Vexc(ac)×Cs/(Cs+Cd)
Note that, a FG-SG coupling part in FIG. 3 is defined for insulating between the frame ground and the signal ground, premising that an impedance thereof is sufficiently low with respect to a frequency of the rectangular wave voltage and that an equation Vfg(ac)=Vsg(ac) can be established. Here, the suffix (ac) indicates an AC voltage component of each signal when viewed from the SG potential (hereinafter, the same in the description).
However, in the case where the capacitance type sensor is used under, for example, a strong noise environment, the relationship Vfg(ac)=Vsg(ac) cannot be established and this results in occurrence of a noise voltage between the frame ground and the signal ground. That is, the frame ground becomes a noise source.
Under such a condition, assuming that the AC voltage component of the FG potential when viewed from the SG potential is Vfg(ac), there appears a term proportional to Vfg(ac) as shown in the following expression, and this component becomes an error.Vdiv=[Vexc(ac)×Cs+Vfg(ac)×Cd]/(Cs+Cd)